Method of producing honeycomb structure body

ABSTRACT

There is provided a method for producing a honeycomb structure, including: a first step of mixing and kneading a ceramic raw material, an organic binder, a water-absorbing resin, and water to obtain clay, a second step of forming the clay into a honeycomb-structured shape and drying the clay to obtain a honeycomb dried body, and a third step of firing the honeycomb dried body to obtain a honeycomb structure having a porosity of 40% or more after firing. The method can suppress defects or deformation upon forming and improve a yield.

TECHNICAL FIELD

The present invention relates to a method for producing a honeycombstructure which can be used for various kinds of filters or the like. Inparticular, the present invention relates to a method for producing ahoneycomb structure, the method being capable of suppressing defects ordeformation upon forming and improving a yield.

BACKGROUND ART

Among various filters, for example, a DPF (diesel particulate filter) isa filter used for trapping and removing particulates contained inexhaust gas from a diesel engine or the like and is incorporated into anexhaust gas system of a diesel engine for use. The filter such as a DPFis produced by bonding a plurality of honeycomb structures (honeycombsegment) with a honeycomb structure being one unit (honeycomb segment).

FIGS. 1 and 2 show a honeycomb structure as one unit (honeycomb segment)to be used for such a DPF. As shown in FIGS. 1 and 2, the honeycombstructure 2 is formed in a cylindrical shape having a square section andhas a large number of cells 5 separated from each other by porouspartition walls 6 inside thereof. The cells 5 extend through thehoneycomb structure 2 in the axial direction, and adjacent cells arealternately plugged with a plugging material 7 at one end portion. Thatis, in one cell 5, the left end portion is open, and the right endportion is plugged with the plugging material 7. In another cell 5adjacent to the above cell 5, the left end portion is plugged with theplugging material 7, and the right end portion is open. Such pluggingforms a checkerwise pattern in each of the end portions of the honeycombstructure 2 as shown in FIG. 1.

Incidentally, a shape of a section of the honeycomb structure 2 may be atriangle or a hexagon besides a square as described above. In addition,a shape of a section of the cells 5 may be a triangle, a hexagon, acircle, an ellipse, or the like.

FIG. 3 shows a DPF as a filter produced by bonding a plurality of theabove honeycomb structures 2. The DPF 1 can be produced by bonding aplurality of honeycomb structure with a bonding material 9 to obtain abonded body, grinding the outer periphery of the bonded body so that asection of the bonded body has a circular, elliptic, triangular, oranother shape, and covering a peripheral surface with a coating material4. The DPF 1 is disposed in a passage for exhaust gas from a dieselengine to trap particulates including soot discharged from the dieselengine.

That is, when the DPF 1 is disposed inside the passage of exhaust gas,exhaust gas flows into the cells 5 of each of the honeycomb structures 2from the left side of FIG. 2 and move toward the right side. Exhaust gasenters from the left side of the honeycomb structure 2 and flows intothe honeycomb structure 2 from the opening cells 5 without beingplugged. The exhaust gas flowing into the cells 5 passes through theporous partition walls 6 and flow out from other cells. When exhaust gaspasses through the partition walls 6, particulates including soot inexhaust gas are trapped by the partition walls 6, and exhaust gas can bepurified.

Such a honeycomb structure 2 can be produced by preparing clay by addingwater to a ceramic raw material and an organic binder as the main rawmaterial to obtain a mixture and kneading the mixture, subjecting theclay to extrusion to have a honeycomb structure, and drying and firingthe honeycomb structure. When particles having low plasticity such as aceramic raw material is used in producing such a honeycomb structure,there arises a problem of insufficient press-bonding at an intersectingpoint of the honeycomb structures due to low plasticity. Incidentally,press-bonding at an intersecting point means a bonding phenomenon ofclay which flows out of grooves from the four direction (left, right,upper, and lower direction) of the extruding die and joins at one pointby being extruded from the extruding die.

When a honeycomb structure having insufficient press-bonding at anintersecting point is used for a DPF, defects are clearly detected by aninspection with laser smoke or the like, and actually cell cracks areobserved. Thus, low plasticity of clay causes a lowered yield.

On the other hand, in a DPF, it is necessary to reduce pressure lossfrom the viewpoint of improving fuel consumption in an engine, and forthis it is required that a honeycomb structure serving as a substrateconstituting a DPF has raised porosity (to increase porosity of thehoneycomb structure). To cope with such a request, there is disclosedthe use of a solid pore-forming material such as a starch or a hollowpore-forming material such as an already foamed forming resin as apore-forming material (see JP-A-2001-373986).

There is also disclosed a method for producing a porous body used as acatalyst support, a synthesis place for various compounds and the like(see JP-A-11-71188). In this production method, a ceramic powder, anorganic binder, and an acrylic acid based resin having highabsorbability are mixed to obtain a mixture, the mixture is extruded togive a formed body, and the formed body is heated and fired. The resinhaving high absorbability has an average particle diameter of 10 to 70μm before absorbing water, several hundreds μm after absorbing water,and a water-absorption ratio of 100 to several hundreds.

Further, there is disclosed a method for producing a porous ceramic usedfor a sensor element, a catalyst support, an incombustible buildingmaterial, a heat-insulating material, a sound-insulating material, ashock-absorbing material, or the like (see JP-A-10-167856). Theproduction method is characterized by having a step of subjecting awater-swelling water-absorbing resin having a gel strength of 10,000dyne/cm² or more to water absorption for gelation, a step of mixing thegel with a ceramic powder for formation, and a step of firing the formedbody. By this method can be obtained a porous ceramic having a porosityof 40% or more and a bending strength of 15% or more of that of a denseceramic of the same component. The water-swelling water-absorbing resinhas an absorbing capacity of 100 to 1,000 g/g (water-absorption ratio of100 to 1,000) for deionized water, and water is not added except forwater absorbed by the water-absorbing resin in this production method.

However, in JP-A-2001-373986, there is inconvenience of causing cracksin a honeycomb structure upon heating for debinding by an excessivetemperature inclination generated in the honeycomb structure due togeneration of heat in a starch. To cope with this, in order toeffectively use an already foamed forming resin as a pore-formingmaterial, it is necessary to make a clay density low for inhibition ofcollapse of the already foamed forming resin during kneading the rawmaterial. However, in the case of making a clay density low, there is aninconvenience of increasing deformation upon forming because hardness ofthe clay is low. Therefore, there arise problems of a lowered yield anddeterioration in size accuracy when only a starch or already foamedforming resin is used as a pore-forming material.

The production method described in JP-A-11-71188 is specifically amethod in which a formed body in the form of a pellet is obtained byextrusion forming, the formed body is granulated to obtain a sphericalformed body, and drying and firing the spherical formed body to obtain aporous body. Though in the method properties of products are notinfluenced by presence or absence of defects upon extrusion forming(formability upon extrusion forming), which is an advantage, there is aproblem that only a product having low porosity of 40% or less can beobtained when the production method is applied to a honeycomb formedbody (see Table 1 of JP-A-11-71188).

Further, since the production method described in JP-A-10-167856 is amethod in which an organic binder is not added, there is a problem oflowering a yield when the method is applied to a honeycomb structurerequiring high plasticity. This is because, a yield can not be improveduntil an organic binder is added in the forming step of a honeycombstructure requiring high plasticity, i.e., a yield can be improved forthe first time by a combined effect by addition of an organic binder anda water-absorbing resin.

The present invention has been made in view of the above problems andaims to provide a method for producing a honeycomb structure, the methodbeing excellent in yield with inhibiting defects or deformation fromgenerating upon forming a honeycomb structure and capable of improvingsize accuracy and having little pressure loss.

DISCLOSURE OF THE INVENTION

In order to achieve the above aim, according to the present invention,there is provided the following method for producing a honeycombstructure.

[1] A method for producing a honeycomb structure, comprising:

a first step of mixing and kneading a ceramic raw material, an organicbinder, a water-absorbing resin, and water to obtain clay,

a second step of forming the clay into a honeycomb-structured shape anddrying the clay to obtain a honeycomb dried body, and

a third step of firing the honeycomb dried body to obtain a honeycombstructure having a porosity of 40% or more after firing.

By such a constitution, a water-absorbing resin mixed and kneaded inclay absorbs water to give a structure in which a resin absorbs water,which has high mechanical strength and hardly collapses. Therefore, evenin the case of making a density of clay high, it has stable poreformability. In addition, since a density of clay can be made high, theclay has high hardness, and deformation upon forming can be suppressedto be very small. Further, by kneading with a ceramic raw material andwater, the ceramic raw material and the water-absorbing resin becomesgranular. Therefore, plasticity of the clay is enhanced, andpress-bonding at an intersecting point can efficiently performed uponextrusion molding. This can inhibit generation of defects. This gives anexcellent yield and can improve size accuracy. Further, thewater-absorbing resin bums out by heating upon debinding, and by theburning out, pores are generated to give a honeycomb structure having aporosity of 40% or more. Thus, by imparting high porosity to a honeycombstructure, pressure loss can be reduced.

[2] A method for producing a honeycomb structure according to the above[1], wherein a resin in a form of particles having an average particlediameter of 2 to 200 μm after absorbing water is used as thewater-absorbing resin constituting the clay in the first step.

This constitution gives excellent size accuracy and can securelysuppress generation of defects by inhibiting pores from becoming largerthan they need after firing.

[3] A method for producing a honeycomb structure according to the above[1] or [2], wherein a resin in a form of particles having a particledistribution of 20 parts by mass or less of particles having an averageparticle diameter of 10 μm or less and 20 parts by mass or less ofparticles having an average particle diameter of 100 μm or more afterabsorbing water is used as the water-absorbing resin constituting theclay in the first step.

By this constitution, clay obtains sufficient plasticity anddispersibility, and pores do not become larger than they need afterfiring. Therefore, generation of defects can be inhibited.

[4] A method for producing a honeycomb structure according to any one ofthe above [1] to [3], wherein a resin in a form of particles having anaverage particle diameter of 30% or less with respect to a thickness ofpartition walls of the honeycomb structure after absorbing water is usedas the water-absorbing resin constituting the clay in the first step.

By this constitution, since pores do not become larger than they needafter firing, generation of defects can be inhibited.

[5] A method for producing a honeycomb structure according to any one ofthe above [1] to [4], wherein a resin in a form of particles having anaspect ratio of 50 or less aster absorbing water is used as thewater-absorbing resin constituting the clay in the first step.

By this constitution, since pores formed by a water-absorbing resinbecome communicating pores after firing, pressure loss can be reduced.

[6 ] A method for producing a honeycomb structure according to any oneof the above [1] to [5], wherein 0.1 to 20 parts by mass of thewater-absorbing resin constituting the clay in the first step is mixedwith respect to 100 parts by mass of the ceramic raw material.

By this constitution, since heat generation upon debinding can besuppressed in the state that plasticity of the clay is enhanced,generation of cell cracks can be inhibited, and a yield can be improved.

[7] A method for producing a honeycomb structure according to any one ofthe above [1] to [6], wherein an amount of the water constituting theclay in the first step is a value obtained by multiplying a mixingamount of the water-absorbing resin by water-absorption ratio (mixingamount of the water-absorbing resin times water-absorption ratio) ormore with respect to 100 parts by mass of the ceramic raw material.

By this constitution, the water-absorbing resin can be made in thesaturated water-absorbing state. Further, an amount of water content fordissolving the organic binder can be ensured, and since a mixing amountof water in the water-absorbing resin is large, porosity of a honeycombstructure after firing can be increased.

[8] A method for producing a honeycomb structure according to any one ofthe above [1] to [7], wherein the water-absorbing resin constituting theclay in the first step is mixed and kneaded in the state that a part ofthe water is previously absorbed by the water-absorbing resin.

By this constitution, the water-absorbing resin absorbs water, and timefor being granulated with the ceramic raw material can be shortened, andas a result, time for kneading can be shortened.

[9] A method for producing a honeycomb structure according to any one ofthe above [1] to [8], wherein a chlorine content in the water-absorbingresin constituting the clay in the first step is 20 parts by mass orless with respect to 100 parts by mass of the water-absorbing resin.

By this constitution, generation of dioxin or the like upon debindingcan be inhibited, which makes a post-treatment step unnecessary andsuppresses a production cost.

[1O]A method for producing a honeycomb structure according to any one ofthe above [1] to [9], wherein a sulfur content in the water-absorbingresin constituting the clay in the first step is 20 parts by mass orless with respect to 100 parts by mass of the water-absorbing resin.

By this constitution, generation of harmful gas such as SOX and H₂SO₄upon debinding can be inhibited, which makes a post-treatment step for adesulfurizer or the like unnecessary and suppresses a production cost.

[11] A method for producing a honeycomb structure according to any oneof the above [1] to [10], wherein a nitrogen content in thewater-absorbing resin constituting the clay in the first step is 20parts by mass or less with respect to 100 parts by mass of thewater-absorbing resin.

By this constitution, generation of harmful gas such as NO_(x), HNO₃ andNH₃ upon debinding can be inhibited, which makes a post-treatment stepfor denitration or the like unnecessary and suppresses a productioncost.

[12] A method for producing a honeycomb structure according to any oneof the above [1] to [11], wherein returned clay is used as the clay inthe first step.

By this constitution, the water-absorbing resin obtains high mechanicalstrength and is hardly collapsed. Therefore, porosity is not deviatedeven if it is used as returned clay, and a yield of the raw material canbe improved.

[13] A method for producing a honeycomb structure according to any oneof the above [1 ] to [12], wherein returned dry clay is used as a rawmaterial containing the ceramic raw material, the organic binder, andthe water-absorbing resin in the first step.

By this constitution, the absorption reaction of the water-absorbingresin becomes reversible, and even if water is removed, similarproperties can be obtained by absorbing water again. Therefore, porosityis not deviated even if returned dry clay is used, and a yield of theraw material can be improved.

[14] A method for producing a honeycomb structure according to any oneof the above [1] to [13], wherein a pore-forming material is furthermixed and kneaded to obtain the clay in the first step.

By this constitution, an amount of the water-absorbing resin to be addedcan be suppressed. Therefore, hardness of the clay is increased, andsize accuracy can be enhanced.

[15] A method for producing a honeycomb structure according to any oneof the above [1] to [14], wherein the ceramic raw material constitutingthe clay in the first step contains as a main component at least oneselected from a group consisting of a cordierite-forming raw material,mullite, alumina, aluminum titanate, lithium aluminum silicate, siliconcarbide, silicon nitride, and metal silicon.

By this constitution, a honeycomb structure can maintain a certain formeven after firing.

[16] A method for producing a honeycomb structure according to the above[15], wherein the cordierite-forming raw material is used as the ceramicraw material, and the water-absorbing resin contains neither alkalimetal nor alkaline earth metal except for magnesium, aluminum, andsilicon.

By this constitution, mixing of alkali metal or alkaline earth metalexcept for magnesium, aluminum, and silicon can be avoided, andextraordinary thermal expansion of the honeycomb structure after firingcan be avoided.

[17] A method for producing a honeycomb structure according to the above[16], wherein metal silicon is used as the ceramic raw material, and adebinding treatment at 500° C. or less for 10 hours or less is conductedbefore the honeycomb dried body is fired in the third step to burn outcarbon contained in the water-absorbing resin.

By this constitution, carbonization of metal silicon can be avoided, andcomposition of the honeycomb structure after firing can be controlled.

[18] A method for producing a honeycomb structure according to any oneof the above [1] to [17], wherein the firing of the honeycomb dried bodyin the third step is conducted in a non-oxidizing atmosphere, and thewater-absorbing resin constituting the clay in the first step does notcontain one or more kinds selected from a group consisting of alkalimetal, sulfur, chlorine, and nitrogen.

By this constitution, mixing of one or more kinds selected from a groupconsisting of alkali metal, sulfur, chlorine, and nitrogen due to thewater-absorbing resin can be avoided, and scattering of these substancesfrom a ceramic formed body to a firing furnace upon firing can beavoided, and thereby inhibiting the firing furnace from being damaged bycorrosion of the furnace material due to scattering of such a substance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of a honeycomb structure.

FIG. 2 is a sectional view taken along with a A-A line in FIG. 1.

FIG. 3 is a perspective view of an embodiment of a DPF.

FIG. 4 is a schematic explanatory view of an inspection apparatus for asoot print test.

REFERENCE NUMERALS

2, 21: honeycomb filter (honeycomb structure), 5, 23: cell, 6, 24:partition wall

BEST MODE FOR CARRYING OUT THE INVENTION

A mode for carrying out a method for producing a honeycomb structure ofthe present invention is hereinbelow described specifically. A honeycombstructure produced according to the present invention has a structureshown in, for example, FIGS. 1 and 2 and is used for, for example, afilter such as a DPF shown in FIG. 3.

A method for producing a honeycomb structure of the present invention ischaracterized by including a first step of mixing and kneading a ceramicraw material, an organic binder, a water-absorbing resin, and water toobtain clay; a second step of forming the clay into ahoneycomb-structured shape and drying the clay to obtain a honeycombdried body; and a third step of firing the honeycomb dried body toobtain a honeycomb structure having a porosity of 40% or more afterfiring.

(First step)

The first step of the present invention is a step where a ceramic rawmaterial, an organic binder, a water-absorbing resin, and water aremixed together and kneaded to obtain clay.

Here, a water-absorbing resin used in the present invention absorbswater when mixed and kneaded with water with the ceramic raw materialand the organic binder described below to have a structure where wateris held inside the resin, having high mechanical strength and beinghardly collapsed. Since the water-absorbing resin and the ceramic rawmaterial are granular when they are mixed and kneaded, plasticity of theclay can be enhanced. In the case of forming a honeycomb structure byextrusion forming using a extrusion die in the first step in such astate as described below, press-bonding at an intersecting point issufficiently conducted. Therefore, formation of defects can beinhibited.

Water-absorption ratio of the water-absorbing resin is preferably 2 to100 times, and more preferably 2 to 50 times. When water-absorptionratio is below 2 times, water-absorbability is low, and sometimesplasticity is not enhanced. When water-absorption ratio is above 100times, because a formed body formed into a honeycomb structure containsmuch water, not only drying time is prolonged, but also much electricpower for drying is required, which sometimes increases a drying cost,and which sometimes lowers a yield because it is prone to be deformed bya lowered hardness of the honeycomb-structured formed body and anincrease in drying shrinkage. Here, shrinkage means an index showing adegree of expansion and contraction before and after drying and can beobtained by (length before drying)/(length after drying). Thus, whenwater-absorption ratio of the water-absorbing resin is 2 to 100 times,plasticity of the clay is enhanced, and a certain hardness ismaintained, thereby giving a honeycomb structure having good formabilityand size accuracy.

Incidentally, a water-absorbing resin described in the aboveJP-A-11-71188 has a water-absorption ratio of 100 to several hundredstimes, and a water-absorbing resin described in the above JP-A-10-167856has a water-absorption ratio of 100 to 1000 times. These resins areclearly different from the water-absorbing resin of the presentembodiment in the point of water-absorption ratio.

In the present embodiment, a resin in a form of particles has an averageparticle diameter of preferably 2 to 200 μm, more preferably 2 to 100μm, after absorbing water. When the average particle diameter is below 2μm, sometimes an effect as a plasticizer is not sufficiently beexhibited. On the other hand, when an average particle diameter is above200 μm, sometimes the honeycomb structure has defects of too large poresafter firing as well as lowered dispersibility due to a relatively largeparticle diameter in comparison with the other powder for use in theclay. When the resin has an average particle diameter of preferably 2 to200 μm after absorbing water, the resin has sufficient plasticity anddispersibility, and pores do not become larger than necessary afterfiring. Therefore, generation of defects can be inhibited.

Incidentally, since the water-absorbing resin described in the aboveJP-A-11-71188 has an average particle diameter of several hundreds μm,the water-absorbing resin is clearly different from the water-absorbingresin of the present embodiment in the point of average particlediameter after adsorbing water.

In the present embodiment, a resin in a form of particles preferablyhaving a particle distribution of 20 parts by mass or less of particleshaving an average particle diameter of 10 μm or less and 20 parts bymass or less of particles having an average particle diameter of 100 μmor more after absorbing water is used, more preferably having a particledistribution of 30 parts by mass or less of particles having an averageparticle diameter of 10 μm or less and 30 parts by mass or less ofparticles having an average particle diameter of 100 μm or more afterabsorbing water, as the water-absorbing resin.

In the particle distribution after absorbing water, when distribution ofparticles having an average particle diameter of 10 μm or less is above20 parts by mass, sometimes an effects as a plasticizer cannotsufficiently be exhibited, and sometimes pore formability is loweredbecause the particles enter in a gap among particles of ceramic rawmaterial. When distribution of particles having an average particlediameter of 100 μm or more is above 20 parts by mass, sometimesdispersibility of the water-absorbing resin is lowered because theaverage particle diameter is larger in comparison with the other rawmaterials. When dispersibility of the water-absorbing resin is lowered,the water-absorbing resin coheres in the clay, and pores formed by thewater-absorbing resin is large after firing, which sometimes becomes adefect in itself. In the particle distribution after absorbing water,when distribution of particles having an average particle diameter of 10μm or less is 20 parts by mass or less and when distribution ofparticles having an average particle diameter of 100 μm or more is 20parts by mass or less, sufficient plasticity and dispersibility isimparted to the clay, and pores are not larger than necessary afterfiring. Therefore, generation of defects can be inhibited.

In the present embodiment, a resin in a form of particles having anaverage particle diameter of preferably 30% or less, more preferably 20%or less, with respect to a thickness of partition walls of the honeycombstructure after absorbing water is used as the water-absorbing resin.

When an average particle diameter of the water-absorbing resin afterabsorbing water is above 30% with respect to a thickness of partitionwalls, a percentage of pores formed by the water-absorbing resinoccupying a thickness of partition walls after firing is high, whichsometimes becomes a defect in itself. When an average particle diameterof the water-absorbing resin after absorbing water is 30% or less withrespect to a thickness of partition walls, pores are not larger thannecessary after firing. Therefore, generation of defects can beinhibited.

In the present embodiment, a resin in a form of particles having anaspect ratio of preferably 50 or less, more preferably 30 or less, afterabsorbing water is used as the water-absorbing resin.

When an aspect ratio of the water-absorbing resin after absorbing wateris above 50, the water-absorbing resin is orientated upon forming ahoneycomb structure, and therefore pores formed by the water-absorbingresin after firing are formed in parallel to the partition walls andhardly become communicating pores, which sometimes causes increase inpressure loss. When an aspect ratio of the water-absorbing resin afterabsorbing water is 50 or less, pores formed by the water-absorbing resinafter firing become communicating pores. Therefore, pressure loss can bereduced.

In the present embodiment, preferably 0.1 to 20 parts by mass, morepreferably 1 to 20 parts by mass, of the water-absorbing resin is mixedwith respect to 100 parts by mass of the ceramic raw material. Thus, itis preferable to determine an amount of the water-absorbing resin mixedin the clay with respect to the ceramic raw material.

When a mixing amount of the water-absorbing resin is below 0.1 parts bymass with respect to 100 parts by mass of the ceramic raw material, themixing amount is too small to enhance plasticity of the clay, and ayield is sometimes lowered. When the mixing amount is above 20 parts bymass, heat generation upon debinding is large, and sometimes a crack iscaused in a honeycomb structure. Thus, by controlling a mixing amount ofthe water-absorbing resin, an amount of heat generation upon debindingcan be suppressed in the state that plasticity of the clay is enhanced.This enables to inhibit generation of a cell crack and to enhance ayield.

In the present embodiment, it is preferable that a mixing amount ofwater is obtained by multiplying a mixing amount of the water-absorbingresin by water-absorption ratio (mixing amount of the water-absorbingresin times water-absorption ratio) or more with respect to 100 parts bymass of the ceramic raw material.

By such a mixing amount of water, the water-absorbing resin can be in asaturated water-absorbing state, and water for dissolving an organicbinder can be secured. This can further enhance plasticity andformability of the clay. In addition, since a large amount of water ismixed, porosity of a honeycomb structure after firing can further beraised.

In the present embodiment, the water-absorbing resin is mixed andkneaded in the state that a part of the water is previously absorbed inthe water-absorbing resin.

By making water to be previously adsorbed, time for granulation of thewater-absorbing resin with the ceramic raw material can be shortened,and as a result, time for kneading can be shortened.

In the present embodiment, a chlorine content in the water-absorbingresin is preferably 20 parts by mass or less with respect to 100 partsby mass of the water-absorbing resin, more preferably not contained.

By thus controlling a chlorine content in the water-absorbing resin,generation of dioxin or the like upon debinding can be inhibited. Whendioxin or the like is generated upon debinding, a post-treatment step isnecessary, and thereby a production cost is increased.

In the present embodiment, a sulfur content in the water-absorbing resinis preferably 20 parts by mass or less with respect to 100 parts by massof the water-absorbing resin, more preferably not contained.

By thus controlling a sulfur content in the water-absorbing resin,generation of harmful gas such as SO_(X) and H₂SO₄ upon debinding can beinhibited when harmful gas is generated upon debinding, a post-treatmentstep for desulfurization or the like is necessary, and thereby aproduction cost is increased.

In the present embodiment, a nitrogen content in the water-absorbingresin is preferably 20 parts by mass or less with respect to 100 partsby mass of the water-absorbing resin, more preferably not contained.

By thus controlling a nitrogen content in the water-absorbing resin,generation of harmful gas such as NO_(x), HNO₃ and NH₃ upon debindingcan be inhibited. When harmful gas is generated upon debinding, apost-treatment step for denitration or the like is necessary, andthereby a production cost is increased.

In the present embodiment, it is preferable that the water-absorbingresin constituting the clay in the first step does not contain alkalimetal, sulfur, chlorine, nor nitrogen when firing of the honeycomb driedbody in the third step is conducted in an inert atmosphere. By thisconstitution, the firing furnace is inhibited from being damaged bycorrosion of the furnace material due to scattering of such a substance.

There is no particular limitation to a ceramic raw material used in thepresent invention as long as a ceramic capable of forming a fixed shapeby firing or a substance which becomes ceramic having a fixed shape byfiring. It is preferable to use as the main component at least oneselected from a group consisting of a cordierite-forming raw material,mullite, alumina, aluminum titanate, lithium aluminum silicate, siliconcarbide, silicon nitride, and metal silicon. By selecting such a rawmaterial, the honeycomb structure can maintain a fixed shape even afterfiring.

It is preferable to use as the main component a cordierite-formingmaterial from the viewpoint of thermal shock resistance. Incidentally, acordierite-forming material means cordierite and/or a raw material whichforms cordierite by firing. As the raw material, there may suitably beselected from talc, kaolin, calcined kaolin, alumina, aluminumhydroxide, and silica, with a chemical composition of 42 to 56 parts bymass of SiO_(2,) 30 to 45 parts by mass of Al₂O₃, and 12 to 16 parts bymass of MgO. In addition, the main component means a substanceconstituting 50 parts by mass or more, preferably 70 parts by mass ormore, more preferably 80 parts by mass or more, of a ceramic rawmaterial.

In the present embodiment, it is preferable that the water-absorbingresin contains neither alkali metal nor alkaline earth metal except formagnesium, aluminum, and silicon when the cordierite-forming rawmaterial is used as the ceramic raw material.

By thus controlling the composition of the water-absorbing resin, mixingof alkali metal or alkaline earth metal except for magnesium, aluminum,and silicon can be avoided, and extraordinary thermal expansion of thehoneycomb structure after firing can be avoided. When mixing of alkalimetal or alkaline earth metal except for magnesium, aluminum, andsilicon is caused, thermal expansion of the cordierite honeycombstructure after firing is increased.

It is preferable to use, as a ceramic raw material, silicon carbidealone or a material containing silicon carbide and metal silicon orsilicon nitride as the main component from the viewpoint of thermalresistance of the honeycomb structure. When the ceramic raw materialcontains metal silicon (Si) and silicon carbide (SiC) as the maincomponents, a Si content is prescribed by a compounding ratio ofSi/(Si+SiC). When the Si content prescribed by the compounding ratio istoo small, it is difficult to obtain an effect of Si addition. When theSi content is above 50 parts by mass, it is sometimes difficult toobtain effect in thermal resistance and heat conductibility. Therefore,the Si content is preferably 5 to 50 parts by mass, and more preferably10 to 40 parts by mass.

In the present embodiment, it is preferable that a debinding treatmentat 500° C. or less for 10 hours or less is conducted before thehoneycomb dried body is fired in the third step to bum out carboncontained in the water-absorbing resin when the metal silicon is used asthe ceramic raw material.

By this constitution, carbonization of metal silicon can be avoided, andcomposition of the honeycomb structure after firing can be controlled.In addition, in the case that debinding at 500° C. or more for 10 hoursor more is required in order to bum out carbon in the water-absorbingresin, oxidation of metal silicon rapidly proceeds.

There is no particular limitation to an organic binder used in thepresent invention, and examples of the organic binder include cellulosessuch as methyl cellulose, hydroxypropoxyl cellulose, hydroxyethylcellulose, and carboxymethyl cellulose and poly(vinyl alcohol). Thesemay be employed alone or in combination. In addition, there may be addeda surfactant such as ethylene glycol, dextrin, fatty acid soap, andpolyalcohol besides the organic binder.

By dissolution of the organic binder, plasticity of the whole clay cansharply be enhanced in cooperation with the effect of increasingplasticity of the water-absorbing resin upon being mixed. By this, ayield can be raised, and size accuracy can be improved. Further, byadding the water-absorbing resin, time for kneading can be shortened,and productivity can be enhanced.

In the present embodiment, it is preferable that a pore-forming materialis further mixed and kneaded in the material in addition to thewater-absorbing resin.

Though the water-absorbing resin itself functions as a pore-formingmaterial, porosity of a honeycomb structure can be raised by furtheradding a pore-forming material. There is no particular limitation tosuch a pore-forming material. Examples of the pore-forming materialinclude graphite, wheat flour, starch, phenol resin, poly(methylmethacrylate), polyethylene, poly(ethylene terephthalate), unfoamedfoaming resin, already foamed foaming resin, shirasu balloon, and flyash balloon. In addition, by using the water-absorbing resin incombination with a pore-forming material, it is possible to suppress amixing amount of the water-absorbing resin. Therefore, hardness of clayis raised, and size accuracy can be enhanced.

In the present embodiment, it is preferable to use returned clay as theclay. Here, returned clay means the clay which is formed again from amaterial subjected to share loading by a kneader, a pug mill, anextrusion die, or the like, via a clay-forming step and a forming step.

By using returned clay as the clay as described above, a yield of theraw material can be raised. Since an already foamed forming resin or thelike has conventionally been used as a pore-forming material whenporosity or a honeycomb structure is raised, it has been difficult touse returned clay as the clay because pore-formability of the clay islowered due to share loading, thereby lowering porosity of a honeycombstructure after firing. When a water-absorbing resin is used as aconstituent of the clay an in the present embodiment, since awater-absorbing resin has high mechanical strength and is hardlycollapsed, porosity of a honeycomb structure after firing is notdeviated even if returned clay is used as the clay. Thus, it is madepossible to use returned clay as the clay, and by using returned clay asthe clay, a yield of a raw material can be raised.

In the present embodiment, it is preferable to use returned dry clay asthe raw material containing a ceramic raw material, an organic binder,and a water-absorbing resin. Here, returned dry clay means the claywhich is to use again, as a raw material containing a ceramic rawmaterial, an organic binder, and a water-absorbing resin, a materialprepared by grinding a dried body prepared by subjecting a material toshare loading by a kneader, a pug mill, an extrusion die, or the like,and drying via a clay-forming step, a forming step, and a drying step.

By using returned dry clay as the raw material containing a ceramic rawmaterial, an organic binder, and a water-absorbing resin, a yield of theraw material can be raised. Since an already foamed foaming resin or anunfoamed foaming resin has conventionally been used as a pore-formingmaterial when porosity or a honeycomb structure is raised, it has beendifficult to obtain similar characteristics by using returned dry clayas the clay containing a ceramic raw material, an organic binder, and awater-absorbing resin because water contained in these resins isremoved, and characteristics of the resins are changed. When awater-absorbing resin is used as a constituent. of the clay, since thewater-absorbing reaction of the water-absorbing resin is a reversiblereaction, it is possible to show a similar level of characteristics byabsorbing water again even if water is once removed. Thus, sinceporosity of a honeycomb structure after firing is not deviated even if amaterial which was made to be returned dry clay as the raw materialcontaining a ceramic raw material, an organic binder, and awater-absorbing resin, a yield of a raw material can be raised by usingreturned dry clay.

(Second step)

The second step of the present invention is a step of forming the clayobtained in the first step into a honeycomb structure, and then dryingto obtain a honeycomb dried body.

There is no particular limitation to a method for forming the clay intoa honeycomb structure, and, for example, an extrusion forming using anextruder may be employed. By thus subjecting the clay to extrusionforming, the clay can be made a formed body having a honeycomb structurehaving a number of cells 5 separated by partition walls 6 and extendingin an axial direction (see FIGS. 1 and 2)

There is no particular limitation to a method for drying the formed bodyhaving a honeycomb structure. For example, hot-air drying, microwavedrying, dielectric drying, drying under reduced pressure, or vacuumdrying may be employed. Among these, it is preferable to employ hot-airdrying in combination with microwave drying or dielectric drying in thatthe whole body can be dried quickly and uniformly.

It is preferable that drying temperature of hot-air drying is within therange from 80 to 150° C. in the point of rapid drying.

(Third step)

The third step of the present invention is a step of firing thehoneycomb dried body obtained in the second step to give a honeycombstructure having a porosity of 40% or more.

There is no limitation to a method for firing a honeycomb dried body.For example, firing in an oxidizing atmosphere, firing in anon-oxidizing atmosphere, or firing in an atmosphere under reducedpressure may suitably be employed.

Since optimal conditions depend on a ceramic raw material used for clay,the firing conditions (firing temperature and firing atmosphere) cannotuniformly be determined. According to a selected ceramic raw material,adequate firing temperature and firing atmosphere can suitably beselected.

For example, when an oxide type of material such as a cordierite-formingraw material or mullite is employed, it is generally preferable to firein an ambient atmosphere. In the case of a cordierite-forming rawmaterial, firing at 1,400 to 1440° C. is preferable. In the case ofnon-oxidizing material such as silicon carbide or silicon nitride,firing in a non-oxidizing atmosphere such as nitrogen or argonatmosphere is preferable. In the case of sintering silicon carbide withmetal silicon, firing at 1,400 to 1,800° C. is preferable. In addition,in the case of sintering silicon carbide with silicon nitride or thelike, firing at 1,550 to 1,800° C. is preferable. In addition, in thecase of sintering silicon carbide particles with each other byrecrystallization method, firing at 1,800° C. or more is preferable. Inthe case of forming silicon nitride by firing metal silicon in nitrogen,firing at 1,200 to 1,600° C. is preferable.

Prior to such a firing treatment, it is preferable to conduct debinding(degreasing) by heating. The debinding treatment can be conducted byheating the honeycomb dried body, for example, at about 400° C. in anambient atmosphere.

Incidentally, in the case of using metal silicon as a ceramic rawmaterial as described above, it is preferable to conduct a debindingtreatment at 500° C. or less for 10 hours or less before the honeycombdried body is fired in the third step to burn out carbon contained inthe water-absorbing resin. In addition, in the case of firing thehoneycomb dried body in an inert atmosphere in the third step, it ispreferable that the water-absorbing resin constituting the clay in thefirst step does not contain one or more kinds selected from a groupconsisting of alkali metal, sulfur, chlorine, and nitrogen.

EXAMPLE

(Examples 1 to 3, Comparative Example 1)

Clay having plasticity was prepared by mixing SiC powder and metal Sipowder as ceramic raw materials, starch and an already foamed foamingresin as pore-forming materials, methyl cellulose andhydroxypropoxylmethyl cellulose as organic binders, and water-absorbingresin A as a water-absorbing resin to give a mixture, adding water tothe mixture, kneading the mixture to obtain clay having plasticity witha vacuum pug mill. There was used water-absorbing resin A having awater-absorption ratio of 10 times and an average particle diameter of50 μm after absorbing water. A compounding ratio of these is shown inTable 1. Incidentally, water-absorbing resin A was not mixed inComparative Example 1.

After subjecting the clay to extrusion forming to give a honeycombstructure, the formed body was dried with microwaves and hot air toobtain a ceramic formed body having a honeycomb structure having athickness of partition walls of 310 μm, a cell density of 46.5cells/cm²(300 cells/inch²), a square section having a side of 35 mm, anda length of 152 mm. The obtained ceramic formed body was measured forperpendicularity, range, and bend, and deformation was evaluated. Theresults of the evaluation is shown in Table 2.

As shown in Table 2, in Examples 1 to 3, where water-absorbing resin Awas mixed, all values of perpendicularity, range, and bend were reducedin comparison with those of the Comparative Example 1, where thewater-absorbing resin was not mixed. Thus, inhibition of deformationupon forming could be confirmed though it was in the middle stage of thepresent invention.

Subsequently, the ceramic formed body was dried with adjacent cellsbeing plugged in mutually opposite end portions so that each end portionformed a checkerwise pattern, degreased at about 400° C. in an ambientatmosphere, and then fired at about 1,450° C. in an Ar inert atmosphereto obtain a Si-bonded SiC segment (honeycomb structure) for a honeycombfilter. The segment was inspected for presence/absence of a defect(frequency of generation) using laser smoke, and kind of defect wasidentified by eye observation. In addition, porosity was measured bymercury penetration. The results of the measurement are shown in Table3.

In the case that any defect is generated in a segment after firing in aDPF production process, the segment was counted as an inferior article,which causes deterioration in yield. Comparative Example 1, where awater-absorbing resin was not mixed, and a raw material of clay has lowplasticity, had a very low yield, and most defects were cell cracks byinsufficient press-bonding due to low plasticity. Example 1, where 0.5part by mass of a water-absorbing resin was mixed, had a greatlyimproved yield. In Examples 2 and 3, where 2 parts by mass and 10 partsby mass of a water-absorbing resin were mixed, respectively, a yield wasfurther improved. Further, “number of cell cracks/number of defects” and“yields due to cell cracks” are shown in Table 3.

(Examples 4 to 9)

A ceramic formed body was prepared in the same manner as in Example 1except that various water-absorbing resins (water-absorbing resins B, D,E, F, G, and H) shown in Table 4 other than water-absorbing resin A wereused. The ceramic formed body obtained was measured forperpendicularity, range, and bend, deformation was evaluated, and thesegment was evaluated for presence/absence of a defect (frequency ofgeneration), number of cell cracks/number of defects, yields due to cellcracks, and porosity. The results of the evaluations and measurementsare shown in Table 4.

In Table 4, the water-absorbing resins B, D, E, F, G, and H havewater-absorption ratios of one time, five times, five times, 50 times,100 times, and 300 times, respectively.

In Example 4, where water-absorbing resin B was employed, thewater-absorbing resin had low water-absorption ratio, plasticity was notsharply enhanced, effect of making a shape better was relatively smallerthan the other Examples, and a yield due to cell cracks was low thoughit was improved. In addition, pore formability was also relatively low,and porosity was 41%, which was relatively low.

In Examples 6 and 7, plasticity was enhanced, and size accuracy wasenhanced. However, since the water-absorbing resin had a large averagediameter after absorbing water, pores formed by the water-absorbingresin were relatively large after firing and became a defect in itself,which caused cell cracks, and a yield was a little lowered. In Example9, an effect in reducing defects by the mixing of the water-absorbingresin was seen. However, since the water-absorbing resin had a largewater-absorption ratio, the honeycomb structure had low hardness, anddeformation was relatively large. In addition, a drying cost was alittle high.

In contrast, in Examples 5 and 8, besides improvement in yield, valuesof perpendicularity, range, and bend were smaller in comparison withthose of Comparative Example 5, and size accuracy was enhanced.

(Example 10)

A ceramic formed body was prepared in the same manner as in Example 5except that a pore-forming material was mixed in addition to thewater-absorbing resin D. The results of the evaluations and measurementsare shown in Table 5. The honeycomb structure obtained in Example 10 hadfurther enhanced size accuracy. TABLE 1 Metal Si Pore-forming Water- SiCpowder powder material absorbing resin compounding compoundingcompounding compounsing Time for Segment ratio ratio ratio Water ratiokneading No. (parts by mass) (parts by mass) (parts by mass) (parts bymass) (parts by mass) (minute) Example 1 1 80 20 15 34 0.5 41 Example 22 80 20 15 49 2 41 Example 3 3 80 20 15 109 10 39 Comp. Ex. 1 4 80 20 1529 — 63

TABLE 2 Perpendicularity Range Bend Segment No. (°) (mm) (mm) Example 11 0.68 0.31 0.40 Example 2 2 0.71 0.41 0.25 Example 3 3 0.71 0.56 0.41Comp. Ex. 1 4 1.22 0.86 0.52

TABLE 3 frequency of Number of cell defect generation crack(s)/ Yielddue to cell Porosity in segment number of crack Segment No. (%) (n =100) defect(s) (%) Example 1 1 54 5 4/5 96 Example 2 2 58 1 0/1 100Example 3 3 65 2 1/2 99 Comp. Ex. 1 4 52 81 74/81 26

TABLE 4 Average particle diameter Frequency Yield after of defect Numberof due to Water- Water- absorbing Perpen- generation cell crack(s)/ cellabsorbing absorption water dicularity Range Bend Porosity in segmentnumber of crack resin No. ratio (μm) (°) (mm) (mm) (%) (n = 100)defect(s) (%) Example 4 B 1 1.7 1.00 0.66 0.42 41 43 42/43 58 Example 5D 5 150 0.67 0.444 0.26 52 6 4/6 96 Example 6 E 5 250 0.58 0.46 0.44 5545 41/45 59 Example 7 F 50 300 0.71 0.49 0.45 63 55 54/55 46 Example 8 G100 100 0.61 0.45 0.40 67 5 3/5 97 Example 9 H 300 150 1.11 0.83 0.54 697 4/7 96

TABLE 5 Pore- Average forming particle material diameter com- FrequencyNumber of Yield after pounding of defect cell due to Water- Water-absorbing ratio Perpen- Poros- generation crack(s)/ cell absorbingabsorption water (parts by dicularity Range Bend ity in segment numberof crack resin No. ratio (μm) mass) (°) (mm) (mm) (%) (n = 100)defect(s) (%) Example 10 D 5 150 10 0.41 0.30 0.15 55 5 5/6 96

(Example 11)

[Method for producing a honeycomb formed body]

As a raw material for aggregate particles, there has been prepared acordierite-forming material having a compounding ratio of 40% by mass oftalc (average particle diameter of 21 μm), 18.5% by mass of kaolin(average particle diameter of 11 μm), 14.0% by mass of alumina (averageparticle diameter of 7 μm), 15% by mass of aluminum hydroxide (averageparticle diameter of 2 μm), and 12.5% by mass of silica (averageparticle diameter of 25 μm).

To 100 parts by mass of the above raw material for aggregate particleswere added 4.0 parts by mass of a water-absorbing resin(water-absorption ratio of 10 times, average particle diameter of 32 μm)as the first pore-forming material and 5.0 parts by mass ofhydroxypropylmethyl cellulose as an organic binder. They were mixed forthree minutes with a ploughshare mixer (Commercial name: PloughshareMixer, produced by Pacific Machinery & Engineering Co., Ltd.). Asstirring conditions of the ploughshare mixer, the ploughshare drivingaxis had a rotational frequency of 100 rpm, and the chopper driving axishad a rotational frequency of 3,000 rpm.

Then, as the second pore-forming material, 1.0 parts by mass of anacrylic resin based microcapsule (average particle diameter of 43 μm)was put in the above ploughshare mixer and mixed for three minutes inthe same manner. Further, there were prepared 0.1 parts by mass of fattyacid soap (potassium laurate) as a dispersant and 55 parts by mass ofwater as a dispersion medium. With spraying and adding a mixed solutionof these in the mixer, mixing was performed for three minutes in thesame manner to obtain a compound for forming (wet powder).

The compound for forming (wet powder) obtained as described above wassubjected to kneading with a sigma kneader and further kneading with ascrew-type extrusion kneader (vacuum pug mill) provided with a vacuumdecompression apparatus to obtain clay extruded into a cylindrical form(outer diameter of 300 mm).

The clay obtained as described above was subjected to extrusion formingwith a ram-type extruder using an extrusion die having slits having ashape complementary to partition walls of a honeycomb formed body toform a honeycomb structure having a number of cells formed and separatedby partition walls. At this time, a screen having a mesh size of 233 μmwas disposed inside the ram-type extruder so that the clay was extrudedafter passing through the screen. The formed body was completely driedby dielectric drying and hot air drying to obtain a honeycomb formedbody. Both end surfaces of the honeycomb formed body were cut off togive predetermined dimensions to the honeycomb formed body.

[Production of a honeycomb filter]

An opening portion of a number of cells of the honeycomb formed bodyobtained as described above was plugged alternately with the otheropening portion to obtain a plugged honeycomb formed body. There wasemployed a plugging method where an adhesive sheet is applied on an endface of the honeycomb formed body, holes were made only in portionscorresponding with cells to be plugged in the adhesive sheet by lasermachining using image processing to give a mask, the end face having themask of the honeycomb formed body was immersed in ceramic slurry to fillup the cells to be plugged of the honeycomb formed body to form pluggedportions.

The ceramic slurry was prepared by mixing, with the same aggregateparticle raw material as in the honeycomb formed body as the aggregateparticle raw material, 0.5 parts by mass of methyl cellulose as abonding agent, 0.3 parts by mass of a special carboxylic acid typehigh-molecular surface-active agent (Commercial name: Poiz 530, Producedby Kao Corporation) as a deflocculant, and 50 parts by mass of waterwith respect to 100 parts by mass of an aggregate particle raw material,stirring the mixture for 30 minutes. The ceramic slurry had a viscosityof 25 Pa•s.

After completely drying the plugged honeycomb formed body obtained asdescribed above by hot air drying, the plugged honeycomb dried body wasfired at 1,420° C. for seven hours to obtain a honeycomb structureconstituted so that admixtures are trapped in the partition walls whenfluid to be treated which is introduced in a part of the cells flows inadjacent cells by passing through porous partition walls.

The honeycomb structure (filter) had a circular end face (cell openingface) having an outer diameter of 229 mm, a length of 254 mm, a squarecell shape of 1.16 mm ×1.16 mm, a thickness of partition walls of 300μm, a cell density of about 300 cells/inch² (46.5 cells/cm²), and thetotal cell number of 19085.

(Comparative Example 2)

A honeycomb structure (filter) was obtained in the same manner as inExample 11 except that there were used 2.0 parts by mass of an acrylicresin based microcapsule (average particle diameter of 43 μm) instead ofa water-absorbing resin as a pore-forming material and 35 parts by massof water as a dispersing medium.

[Evaluation]

With regard to the honeycomb structures (filters) in Example 11 andComparative Example 2, evaluation was given on an extent of internaldefects of the honeycomb structure (filter), i.e., filtrationperformance (trapping efficiency) of the filter by calculating asoot-leaking cell ratio by a soot print test.

The soot print test was conducted using, as shown in FIG. 4, aninspection apparatus 31 constituted by a support 32 for supporting ahoneycomb structure (filter) 21 in the state that the peripheral edgeportions are airtightly sealed, a soot generator 34 joined to thesupport 32 and supplying gas containing graphite particles, a screen 36(using white cloth) for trapping graphite particles, and exhaust gaspipe 38 for collecting gas passed though the screen 36 according to themethod described in JP-B-5-658.

First, each of honeycomb structures (filters) 21 in Example 11 andComparative Example 2 was mounted on the support 32, and the exhaust gaspipe 38 was set on the upper end face of the honeycomb structure(filter) 21 to fix the honeycomb structure (filter) 21 in the state thatthe honeycomb structure (filter) was held between the support 32 and theexhaust gas pipe 38. In this state, gas containing graphite particleswas sent into the cells from one end face side of the honeycombstructure (filter) 21 from the soot generator 32 at about 70 g/hour toaccumulate soot at 5 g/liter in the honeycomb structure (filter) 21. InFIG. 4, the reference numerals 22 and 24 denote plugged portions andpartition walls, respectively.

Next, after sticking the screen 36 fast to the upper end face of thehoneycomb structure (filter) 21, the exhaust gas pipe 38 was set againfrom above the screen 36 to fix the honeycomb structure(filter) 21 andthe screen 36 in such a state that they were held between the support 32and the exhaust gas pipe 38. In this state, gas containing graphiteparticles was sent into the cells from one end face side of thehoneycomb structure (filter) 21 from the soot generator 32 at about 70g/hour, and images (i.e., black points) of graphite particles trapped bythe screen 36 having gas permeability and stuck fast to the other endface were observed, and the number was counted.

As a result, the honeycomb structure (filter) obtained in Example 11 hada soot-leaking cell ratio of 0.5 cell/1,000 cell (i.e., 1 cell/1,000cell or less) with few internal defects and excellent filtrationperformance (trapping efficiency). In contrast, the honeycomb structure(filter) obtained in Comparative Example 2 had a soot-leaking cell ratioof 2.5 cell/1,000 cell (i.e., 1 cell/1000 cell or less) with not a fewinternal defects and insufficient filtration performance (trappingefficiency).

Incidentally, each of honeycomb structures (filters) obtained in Example11 and Comparative Example 2 was cut at a portion where soot leakage wascaused, and the portion was observed. A few small pores having adiameter of about 0.5 mm were observed in a honeycomb structure (filter)obtained in Example 11, which is allowable as an extent of internaldefects. However, not a few fine sprits or cuts having a length of about10 to 100 mm were observed in a honeycomb structure (filter) obtained inComparative Example 2, which is beyond the allowable level and an extentof internal defects.

From this, it can be said that the water-absorbing resin enhancedflowability, press-bondability, etc., of clay for forming the honeycombstructure and reduced internal defects of the honeycomb structure.

Industrial Applicability

A method for producing a honeycomb structure of the present invention iseffectively used in various industrial fields requiring various filtersfor a diesel engine exhaust gas-treating apparatus, a dust-removingapparatus, a water-treating apparatus, etc.

1. A method for producing a honeycomb structure, comprising: a firststep of mixing and kneading a ceramic raw material, an organic binder, awater-absorbing resin, and water to obtain clay, a second step offorming the clay into a honeycomb-structured shape and drying the clayto obtain a honeycomb dried body, and a third step of firing thehoneycomb dried body to obtain a honeycomb structure having a porosityof 40% or more after firing.
 2. A method for producing a honeycombstructure according to claim 1, wherein a resin in a form of particleshaving an average particle diameter of 2 to 200 μm after absorbing wateris used as the water-absorbing resin constituting the clay in the firststep.
 3. A method for producing a honeycomb structure according to claim1, wherein a resin in a form of particles having a particle distributionof 20 parts by mass or less of particles having a particle diameter of10 μm or less and 20 parts by mass or less of particles having aparticle diameter of 100 μm or more after absorbing water is used as thewater-absorbing resin constituting the clay in the first step.
 4. Amethod for producing a honeycomb structure according to claim 1, whereina resin in a form of particles having an average particle diameter of30% or less with respect to a thickness of partition walls of thehoneycomb structure after absorbing water is used as the water-absorbingresin constituting the clay in the first step.
 5. A method for producinga honeycomb structure according to claim 1, wherein a resin in a form ofparticles having an aspect ratio of 50 or less after absorbing water isused as the water-absorbing resin constituting the clay in the firststep.
 6. A method for producing a honeycomb structure according to claim1, wherein 0.1 to 20 parts by mass of the water-absorbing resinconstituting the clay in the first step is mixed with respect to 100parts by mass of the ceramic raw material.
 7. A method for producing ahoneycomb structure according to claim 1, wherein an amount of the waterconstituting the clay in the first step is a value obtained bymultiplying a mixing amount of the water-absorbing resin bywater-absorption ratio (mixing amount of the water-absorbing resin timeswater-absorption ratio) or more with respect to 100 parts by mass of theceramic raw material.
 8. A method for producing a honeycomb structureaccording to claim 1, wherein the water-absorbing resin constituting theclay in the first step is mixed and kneaded in the state that a part ofthe water is previously absorbed in the water-absorbing resin.
 9. Amethod for producing a honeycomb structure according to claim 1, whereina chlorine content in the water-absorbing resin constituting the clay inthe first step is 20 parts by mass or less with respect to 100 parts bymass of the water-absorbing resin.
 10. A method for producing ahoneycomb structure according to claim 1, wherein a sulfur content inthe water-absorbing resin constituting the clay in the first step is 20parts by mass or less with respect to 100 parts by mass of thewater-absorbing resin.
 11. A method for producing a honeycomb structureaccording to claim 1, wherein a nitrogen content in the water-absorbingresin constituting the clay in the first step is 20 parts by mass orless with respect to 100 parts by mass of the water-absorbing resin. 12.A method for producing a honeycomb structure according to claim 1,wherein returned clay is used as the clay in the first step.
 13. Amethod for producing a honeycomb structure according to claim 1, whereinreturned dry clay is used as a raw material containing the ceramic rawmaterial, the organic binder, and the water-absorbing resin in the firststep.
 14. A method for producing a honeycomb structure according toclaim 1, wherein a pore-forming material is further mixed and kneaded toobtain the clay in the first step.
 15. A method for producing ahoneycomb structure according to claim 1, wherein the ceramic rawmaterial constituting the clay in the first step contains as a maincomponent at least one selected from a group consisting of acordierite-forming raw material, mullite, alumina, aluminum titanate,lithium aluminum silicate, silicon carbide, silicon nitride, and metalsilicon.
 16. A method for producing a honeycomb structure according toclaim 15, wherein the cordierite-forming raw material is used as theceramic raw material, and the water-absorbing resin contains neitheralkali metal nor alkaline earth metal except for magnesium, aluminum,and silicon.
 17. A method for producing a honeycomb structure accordingto claim 15, wherein metal silicon is used as the ceramic raw material,and a debinding treatment at 500° C. or less for 10 hours or less isconducted before the honeycomb dried body is fired in the third step tobum out carbon contained in the water-absorbing resin.
 18. A method forproducing a honeycomb structure according to claim 1, wherein the firingof the honeycomb dried body in the third step is conducted in anon-oxidizing atmosphere, and the water-absorbing resin constituting theclay in the first step does not contain one or more kinds selected froma group consisting of alkali metal, sulfur, chlorine, and nitrogen.